Ground potential compensator



2 sheets-snaai 1 G. C. CUMMINGS GROUND POTENTIAL' COMPENSATOR Filed April 7, 1939 HI' 'n -w May 7, 1940 /N VEA/TOR G. C. CUMM/NGS er l A rrQR/VEV May 7, 940 G. c. cvuMMlNGs GROUND POTENTIAL COMPENSATOR Filed April '7, 1939 2 Sheets-Sheet 2 UY QQ /NVENTOR G. C. CUMM/NGS N .Ql

ATTORNEV Y Patented May 7, 1940 UNITED STATES GROUND POTENTIAL COMPENSATOR George CrCummings, Orange, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application April 7, 1939, serial No. 266,666

9 Claims.

This invention relates to regulating systems and particularly to devices used in communication systems for maintaining the potential at one point substantially equal to the potential at 5 a distant point so that communication between such points will not be disturbed by ground potentials. l Y

It is usual to employ an artificial ground connection and to regulate this in accordance with the potential of the. earth at a distantl oiiice. Heretofore, direct current generators have been employed and means controlledA from the distant oice have been used to excite the elds of these generators.

The object of the present invention is to improve this general arrangement by regulating the potential of the artiiicialor compensated ground by supplying to it either plus or minus direct current by rectifying alternating current. 20 Since the ground potential at the distant control point may shift rapidly and from one extreme to another, the rectifying means here used must be equally rapidly shifted. 4-

A feature of the invention is apair of rectiiying tubes, one for supplying direct current in one ldirection and the other for supplying direct current in the opposite direction which may be rapidly controlled so that oneat a time is effective and over a wide range. l

Another feature of the invention is'an alter- 30 nating current phase shifting bridge circuit for controlling the rectifying tube. When two such bridge circuits are used, one to control each rectifying tube, the transition from operation at peak value of one rectiiying tube through nonoperation of both tubes to operation of the other may be smooth and rapid and, in fact, may be instantaneous. l Y i I In accordance with another feature of the invention, a potentiometer circuit is formed between the control ground and the compensated ground and a vacuum tube amplifier is employed to amplifypotential changes as they may occur to in turn control thephase shifting bridge circuits.

In one form of the' invention, the rectified alternating current is `used to excite a ground compensating generator. In another form, the rectified alternating current is fed directly into the compensated ground Vwhereby the delay in following the potential changes due to inductance in the generator field circuits is avoided. With compensation carried out by vacuum tube circuits alone, the changes are practically instantaneous and the compensation is, therefore, more nearly perfect than it has heretofore been possible to attain.

The drawings consist of two sheetsthe iirst containing Fig, land the second containing Figs. aand 3.

Fig. 1 is a circuit 'diagram illustrating the peferred embodiment of the present invention; and Figs. 2 and 3v are skeleton circuit diagrams illustrating variations of vthe invention, only Venough of the circuits being .shown t'o explainf such variations.

Fig. 1 shows one embodiment of the invention. Here is shown an 'office ground 'I which, because of some resistance to ground, is unsatisfactory as a connection for communication circuits eX- tending t'o a distant point. I At some other lpoint there is another ground connection, here designated as a control ground 3.Y Between the oce groundl I and other distant stations there exists a varying groundpotential. To'overcome the eiects of this and render the operation of communication circuits between these points stable, a compensated ground 2 is provided and the circuits oi this figure illustrate Aa; means for maintaining the potential of the compensated ground 2 constantly at the same level as the potential of thev control ground 3 which' is so chosen as to have zero ground potential.`

Essentially, this means consists of a source of rectified alternatingcurrent connected between the oiiice groundt'lf andA thecom'pensated ground 2 by rectifier tubes'. Certain pieces of electrical apparatus, suchasim'peda'nce coil`8, condensers 9 and ballast or tungsten"iilarnent lamp II), constitute a low-passv iilt'err` wherebythe rectied alternating current is eiiectively transformed into a direct current potential 'to raise or lower the potential of the compensatedground 2 above or below the potential of the ,office ground I. The amount by which .the potential of the compensated ground 2 is varied., is controlled by a vacuum tube circuit inaccordance with the v ariations of the potential of the distant control ground 3. l

This 'control is exercised byvirtue of a connection from the contro-l. ground 3 to the coinvpensated ground v2, comprising conductor 4, po-

tentiometer 1, conductor 5, and impedance coil 8. The conductor II is shown extending through certain resistances 6 'and condenser connections toqground to illustrate the normal condition of a long line, which in some casesfmay be of the order of a hundred miles. Eroma point on the potentiometer 'I there is a connection to a vacuum tube II which :actsasga simple amplifier. This ieeds` into another vacuum tube I2 which Vin turncontrols a pairof Vacuumtubes I3 and I4. Under normal balancedconditions, the tube I2 maintains the grids of vacuuinj tubes I3 and I4 both at the same potential, but when a change takes place the potential of theggrid of tube I3 is raised or lowered, while the potential of the grid of tube I 4 iscorrespondingly lowered or `raised by the same amount-f The tubes, I3 andA I4.,v formy variable elements coils 29 and 30, to a similar transformer associated with tube I4 and to a transformer having a primary winding 33 and a secondary winding 32. inafter, this alternating current connection will result in the ilow of rectified current from the compensated ground 2 through the anode-cathode circuit of tube I5 to the oice ground I over a range from a maximum Value to Zero or, similarly, from the ofce ground I through the anodecathode circuit of tube I6 to the compensated ground 2 from a maximum value to zero.

The action of the alternating current phase shifting bridge and the control of the rectifier tubes is well known, and is described andthe theory of operation is explained, for example, in an article entitled Hot-cathode Thyratrons in the General Electric Review, vol. 32, No. 7, for July, 1929.

The tubes II to IB are shown with heater laments unconnected to any circuit, but it will be understood that appropriate connections are made to the source of power 34. Such connections are omitted here for the sake of clarity in the drawings.

The condensers 9 are shown as a pair since they are of large capacity and in practice they would be of the electrolytic type. The lamp III indicates a ballast lamp which under practical conditions may be a tungsten filament electric lamp having a negative coecient of resistance. This oiers a path of low impedancek between the oce ground I and 'the compensated ground `2 under normal conditions when the potential of the compensated ground 2 is the same as the potential of the control ground 3 but will raise this impedance as current begins to flow therethrough.

In more detail, the constants of the tube i I and its circuit are so adjusted that, when no potential difference exists between the compensated ground 2 and the control ground 3, the tube is working at about the mid-point of its characteristic curve and at this point the constants of the other tubes and their circuits are fixed so that there is no current now from the ofce ground I to or from the compensated ground 2 and through the anode-cathode circuits of tubes I5 and I6. The arm I'I of the potentiometer 'I is so adjusted that if a diierence of potential between the compensated ground 2 and the control ground 3 is set up, the potential of the grid of tube II becomes more positive or more negative. Since tube II acts as a simple amplifier, a plate current will flow through resistance 20, part of resistance ZI, conductor 5, part of resistance I9, to the cathode of tube II. The resistance 20 is in a circuit in which current flows from positive battery through resistance I8, resistance 20, part of resistance 2|, conductor 5 and thence back to compensated ground. The variation in the current flow in resistance 2D, caused by theaction of tube II, causes a change in the potential of tube I2 which is a corresponding and amplified change to that on the grid of tube Il.

Tube II is shown as a pentode with its plate connected to plus battery through a resistance I8 and its screen connected directly to battery. It will-be understood, however, that the use of a pentode is not necessaryas an equal result can be obtained from other types of tubes.

As will be explained more in detail here-. V

Tube I2, then, is controlled in the same Way as tube II, the latter acting as a simple amplifier of they change of potential on its grid. Tube I2 functions to control the potentials of the tubes I3 and I4. Under normal conditions these two grids are maintained at the same potential but as tube I2 is aiected so are the grids of tubes I3 and I4. When the potential of the one is raised the potential of the other is lowered by an equal amount.

The plate of tube I2 is connected through a resistance 23 to positive battery, while positive battery is connected directly to the screen. From positive battery a circuit may be traced through the resistance 23, the potentiometer formed by resistances 24 and 25 to negative battery. Likewise, a circuit may be traced from negative battery through the potentiometer formed of resistances 2B and 2'I through the resistance 2I to compensated ground. These resistances are all so chosen that, when there is no diierence of potential between the compensated ground 2 and the control ground 3, the potentials on the 'grids of tubes I3 and I4 are equal. Then if the potential applied to the grid of tube I2 is varied the plate current of tube I2 varies and, due to the potentiometer connections to the grids of tubes I3 and I4, these tubes are affected, equally but in opposite directions. Ii the potential applied to the grid of tube I2 becomes more positive the potential of the grid of tube I3 is reduced by an increased drop across the resistance 23 and the potential of the grid of tube I4 is increased by an increased drop across resistance 2l. If, on the other hand, the potential applied to the grid of tube I2 becomes more negative then the potential of the grid of tube I3 is increased by the reduced drop across resistance 23 4and the potential of the grid of tube I4 is reduced by the reduced drop across the resistance 2|. Thus the action of the tubes I3 and I4 is equal and opposite.

The anode-cathode circuit of tube I3 forms an adjustable arm of an alternating current phase shifting bridge, the other three arms being the condenser 23, the upper half 29 and the lower half .3K1 of the secondary winding of a transformer having a primary winding 3 I. The tube I5 is connected between diagonal points of this bridge, the grid of tube I5 being connected by means of the isolation transformer 35 to the point between transformer windings 29 and 30 and the anodecathode circuit being connected to a point between the cathode of tube I3 and condenser 28. By means of the variation of the flow of current between the anode and cathode of tube I3, the grid of tube I5 is so aiected that a perfect control of average current may be obtained in the circuit between the olce ground I and the compensated ground 2 from zero to a maximum. Since the variation of the flow of current between the anode and cathode of tube I3 varies the phase of the grid of tube I5 with respect to its anode-cathode circuit, the tube I5 is rendered ineffective in varying degree.

In like manner but in an opposed direction, tube I4 controls tube I6 with the net result that the direct current potential set up on the' compensated ground 2 through the action of tubes I5 and I6 may be smoothly changed from a maximum in one direction through zero to a maximum. in the other direction.

Due to the great capacity of condensers 9 and the fact that the response of the vacuum tubes is extremely quick, the potential of the compensated ground 2 may be kept practically at a level with the potential of the control groundl 3. Any

change in this potential immediately results in a great flow of current to quickly charge the condensers 9 in one direction or the other to maintain the potential level of the compensated ground 2.

Figs. 2 and 3 show variations of the invention. Fig. 2 shows how the output of the tubes l5 and I6 may be applied to the field 35 of a direct current generator 35 running at a constant speed. The reaction of such a device can be made suiiciently rapid to provide reasonably satisfactory compensation though the arrangement of Fig. l is preferred.

Fig. 3 shows how the output of tubes l5 and i6 may separately be applied to the windings 31 and 38 of a double eld for generator 36. It is believed that the action of these arrangements will `be fully understood in connection with the description of Fig. 1.

What is claimed is:

l. A ground connection for electrical communication systems comprising an ofce ground, a compensated ground, a control ground, a source of alternating current, rectiying means and a space discharge tube amplifier under control of said control grounds for controlling the ilo-w of alternating current from said source rectied by said rectifying means between said oice ground and said compensated ground in direction and volume suflicient to maintain said compensated ground at substantially the` same potential as said control ground.

` 2. A ground connection for electrical coinmunication systems comprising an office gro-und,

' a compensated ground, a control ground, a

source of alternating current, rectifying means, a space discharge tube circuit for feeding direct current rectied from said alternating current source from said office ground into said cornpensated ground and a connection from said control ground to said discharge tube circuit for adjusting the flow of said current in said discharge tube circuit over a predetermined range on either side of zero for automatically maintaining the potential of said compensated ground at the same level as said control ground.

3. A ground connection for electrical communication systems comprising an office ground, a compensated ground, a control ground, a source of alternating current, and rectifying means connected between said oilice ground and said compensated ground, a potentiometer connection between said compensated ground and said control ground and a space discharge tube control circuit controlled from said potentiometer circuit for controlling the flow of rectied alternating current in direction and volume su'icient to maintain said compensated ground at substantially the same level as said control ground.

4. A ground connection for electrical communication systems, comprising an office ground, a compensated ground, a control ground, a source of alternating current and a pair of rectifying tubes, one for directing current in one direction and the other for directing current in the reverse direction connected between said oice ground Aand said compensated ground, Va potentiometer connection between said compensated ground and said control ground and a space discharge tube circuit controlled from said potentiometer circuit for regulating said rectifying tubes for automatically maintaining said compensated ground at substantially the same potential as said control ground.

5. A ground connection for electrical com-r munication systems, comprising an cnice ground, a compensated ground, a control ground, a source of alternating current, a discharge tube rectifier circuit for feeding direct current rectified from alternating current from said source between said omee ground and said compensated ground, an alternating current phase shifting bridge circuitl for controlling said rectifier circuit, and means for controlling said alternating current phase shifting bridge circuit from said control ground.

'6. A ground connection for electrical communication systems comprising an oiiice ground, a compensated ground, a control ground, a source of alternating current, a connection between said oiice ground and said compensated ground including a connection from said source of alternating current and the anode-cathode circuits of two space discharge tubes oppositely connected in parallel with each other, an alternating current phase shifting vacuum tube bridge circuit for controlling each of said discharge tubes, and means for controlling said alternating current phase shifting vacuum tube bridge circuits in inverse relation to each other both from said control ground. l

7. A ground` connection for electrical communication systems, comprising an ofiice ground, a compensated ground, a control ground, a source of alternating current, rectifier means for feeding direct current rectified from said alternating current over a range from a predetermined plus `value through zero to a predetermined minus value into said compensated ground, and a pair of alternating current phase shifting discharge tube bridge circuits, one directly and the other inversely responsive to potential changes in said control ground for controlling said rectier means.

8. A ground connection for electrical communication systems comprising an cnice ground, a compensated ground, a control ground, a direct current generator having its armature connected between said cnice ground and said compensated ground, a source of alternating current, rectifying means for energizing the field of said generator from saidsource of alternating current, and a pair of alternating current phase shifting discharge tube bridge circuits,l one directly and the other inversely responsive to potential changes in said control ground for controlling said rectifying means.

9. A ground connection for electrical communication systems comprising an olice ground, a compensated'ground, a control ground, a direct current generator having its armature connected between said oflice ground and said compensated ground, a source of alternating current, rectifying means for energizing the eld of said generator from said source of alternating current, a pair of alternating current phase shifting space discharge tube bridge circuits, one directly and the other inversely responsive to potential changes in said control ground for controlling said rectifying means, said generator eld comprising two normally opposed windings and said rectifying means comprising a space discharge tube controlled by each of said alternating current phase shifting bridges for separately exciting said neld windings. 

